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Topic: Mad strobe? (Read 7320 times)

Hi to old friends! Sorry I've been away so long -- one of my periodic lapses! I've been building bows, so only woodwork this winter.

Lately I was thinking I'd really like to see what a bow and arrow was doing when it releases. I'd like to know what changes in design do to arrow speed at multiple moments during release. There are high speed consumer grade video cameras out there made by Casio that can shoot 1000 fps -- but the quality of the video isn't too great, and the price for a used one imported from japan is $120 -- on up. Unsure of quality and out of my price range at present.

I don't know what's available super cheap for multiple flash stop-action strobes either, but I suspect they aren't cheap. They'd have to charge a pretty big capacitor (I assume) to flash like 10 frames in maybe a 50th of a second. And some kind of programmable sophisticated interval timer circuit.

So then I was thinking of good old Muybridge with his multiple cameras fired by strings across a race track way back in photo history. Only could I do something similar with say, a bunch of old fashioned hot shoe camera flash units from ebay triggered by an Arduino board. I was thinking I could program in the intervals wanted, that way.

Does this seem feasible? I don't remember exactly how the flash shoe triggering worked, but I think it was just a switch -- is that right? Or was there a signal from the camera? "Hot" shoe does seem to imply a voltage. Anybody know specs if so?

I figure the time interval of interest -- might not be totally accurate, but here it is.... A very fast arrow from a wooden bow might reach 200 feet per second terminal velocity, and is about 28" long. Since it starts from zero, it averages about 100 ft/sec through the bow, and is about 2 feet long -- so as a guess roughly 1/50 second in the bow. Let's say 1/60th to be more conservative -- I don't know actually.

It would be nice to have an image every tenth of that -- ten images total -- so the intervals between flashes might be about 1/600 second. The flash duration itself should be as short as possible. I don't know how short a used flash unit can flash for, but a tenth of the interval would be nice. That's a duration of say 1/6000th second. No idea if that is possible -- anybody know?

That would mean the arrow traveled about 1/4" (if I'm figuring right) during the duration of the flash. At that rate the phot will be a little blurry, but maybe good enough to get a measurement of the distance from release, which is the main thing I'm interested in. Faster would obviously be better -- if you think that's possible....?

I think most of the automatic thyristor flashes for SLRs of the past adjusted the flash length -- which isn't something I want. Ideally it would just flash for one set duration, and flash as short as possible to still register on my digital camera set to a time exposure. The bow would be set against a black background and the distance to subject would be quite short. The arrow would have an automatic release.

I'm thinking an Arduino Uno R3 board (I have a few) and an interface digital switching board like this 8 channel job (well I guess that's 8 frames per photo, but acceptable). But I don't know if it would be fast enough:

Tucked away in a cupboard somewhere I have a stand alone strobe unit that can be varied from several seconds between flashes, up to freezing rotation of a part spinning at several thousand RPM. Uses a triggered neon tube iirc. They turn up on eBay for peanuts. The spectrum of the flash might be an issue, I presume that it's at the red end. Not sure if it would match the response of a digital camera too well?

Most of the work in this field has been done and can be found on the Web. Basically the 2D sinusoidal path of the Arrow is complicated by changes in; Weight, Weight distribution, Length, Fletching length, Fletching height, Young's Modulus relating to the Shaft just for starters. Then there is the Resonant Frequency of the Bow & BowString to consider ... When we were playing around with all of this, we just gave up, (as it was doing our Heads in), and concentrated on what happened at the Target ...

Hey Andrew! Do you have a mfr and type for something like that -- looking for something roughly $50 range. Not sure what to look for. Red is okay if it isn't infra-red out of the digital camera's usable spectrum. I'm not looking for fidelity, just position.

imagineering, not interested in the arrow flight or its physical nature, other than mass. An unfletched lumberyard dowel is fine. Only what the arrow and limbs are doing during the power cycle is of interest, as I said above. I'm building bows, not arrows. And what is of interest is seeing what an incremental change I make on a bow yields in actual photographed effects, not following predictions from somebody else's mathematical model. And yes the latter may be all over the net, but they change every few years and are the subject of continuing arguments.

I don't want a formula for someone else's bows built to their ideas of efficiency, I want time lapse photos of my own bows built to what I want out of a bow. Turns out my old eyes aren't good enough to record and memorize 8 images in a 60th of a second...

Steve, mine was made yonks ago - probably in the 1950's from the black crackle paint finish - by Ferranti who I worked for for many years. I've seen the same design made by several of the defence companies so I suspect that it was an NPL design (National Physical Laboratory)

It used a tube like this one currently featuring on eBay, and is just a relaxation oscillator. Capacitor charges via a variable resistor, until the tube fires and discharges it.

Thanks Andrew, I see. Well I was hoping for something either pre-built, cheap and available, or this other project cobbing together ordinary flash units and an arduino without too much circuit engineering/building. I think I actually already own a couple of hot shoe flashes from the old, and older days, and definitely have the arduino.

Maybe I'll plunk for the solid state relay unit, and try it out with my couple of flashes "just to see". Unless somebody has a better idea -- or warns me off of that relay.

Still curious if anybody knows how a hot shoe flash is triggered -- just a closed contact, or is is a voltage signal....?

Do you need to have actual images of the arrow during its flight? It occurs to me that if you had some sort of baffles set up on either side of the "shooting range" (imagine a alleyway made up of walls of cardboard) than you could use an Arduino to record precise intervals of time as the arrow's flight triggered a series of either ultrasonic or IR sensors. Since you'd know ahead of time the distances at which the sensors had been placed, you could easily map the time difference between one trigger event and the next. Or, you could open a whole different can of worms, and look into making a radar trap out of an old microwave.

I know that this video is not what you're looking for, but it is fairly on-topic:

Josh thanks. No it's not the arrow in flight I want to strobe-photo, it is the bow string on the arrow (in the nock), and the bow limbs, during the brief period before the arrow has left the bow.

This period of interest is about 1/60th of a second. And I'd like to divide it into 8 to 10 images so I can measure the acceleration of the arrow through the bow by measuring the distance the knock advances per frame.

I'm seeing some stroboscope projects on Instructables using LED arrays instead of the camera's usual flash tube. I'm wondering if they would prove bright enough. Here's one well documented project:

it uses a stamp instead of an arduino, and analog inputs for rate and pulse duration. I'd probably use an arduino and just program in the other parameters, so I could maybe simplify the circuit quite a bit. I'd only need one output channel instead of 8 to 10 with multiple flash strobes. The only question is whether the LED array would be bright enough.......

Way back when digital cameras were just starting to make film obselete, I was still playing with film. I recall sitting in my darkened closet, camera on a tripod, party strobe lite going nuts, and a white pendulum swinging back and forth in front of the lens.

The comments about how I spent my free time aside, it did actually work and the pictures showed the pendulum in multiple positions in the same exposure - which was rather long. You can probably do the same thing with a DSLR in manual mode. Is it a safety hazard firing an arrow in complete darkness with a strobe light going? Sounds like a fun party!

I do like the idea of the engine timing light too. You could possibly trigger it with a microcontroller to get the timing accurate. There might be an upper limit on recharge rate though. An auto-relay (might be called something different) setup like the old Model T "buzz-box" could be used to excite an auto ignition coil and you could trigger your timing light off that. You might be able to find a place to measure the frequency to make sense of the picture in post processing.

Ya know I do have an old auto timing light! Sears, I think. I think it fired off an inductive clip over the plug wire didn't it? It was white light -- I don't think it was neon orange.

Anyway I sent for some 12 V high power 11 watt LED panels --ten for $19. I guess we'll see if 100 watts of LEDs is enough. I heard that you could up the current if it was low duty cycle like this is, but I have a feeling it might work without.

I also came across a cool reference to spinning a black disk 600 rpm with a slot in it in front of the camera lens for a high speed multiple shutter. Then you just need a lot of light on the subject. That would solve the black cabinet problem because outdoors has a lot of light, and you could hand fire the bow. That might make the most sense. But the Arduino and LEDS seems like a fun experiment anyway. I already have the Arduino.

I was thinkking of a 1kW halogen light on a well ventilated box, opening on the front and pretty close to it a spinning shutter disc AC induction motor connected to a mains frequency (specially on three phase) should produce pretty accurate timing.

Then you need a SLR type curtain to run trough line of the light...ever wanted to build a 100x SLR shutter

If you build a plate shutter that drops by gravity and it is released with a solenoid, simply wired to your string release trigger.

That should produce completely "mechanical" solution. You need huge amount of light to "stop" motion. We had a high speed camera and we needeed room filled with a light, it was so bright that sunglasses were a must.

Pekka, I think the cooler LED's fully illuminating the subject with each pulse will be easier to do, plus I'd have control over all kinds of things like number of pulses, duration, frequency -- even non-linear pulsing -- logarithmic variation, for instance.

I better give you-all the point of this. Mainly to understand how much energy the arrow is getting from the limbs and string at intervals along the path through the bow. Nobody can predict that for sure. And it depends on the inertia of the arrow and the speed of the limbs. If the arrow has low inertia (a light arrow) and the limbs aren't fast enough, it's possible that the arrow gains energy only in the early part of the cycle or even outruns the string. Bows also don't act as uniform springs -- their rate, draw weight, and internal resistance (hysteresis) varies along the draw length. If a bow pulls back easy at first but increases rapidly near full draw, that is called "stacking". A high stacking bow and a light arrow will behave differently than a bow that stacks less or an arrow with more mass that accelerates slower.

Anyway I want to see all this, and stop listening to endless internet conjecture about it. A picture is worth a thousand words,as they say, and a lot of pictures must be worth even more!

Sounds a lot like BS about fly rod "action" and other properties long time ago. And there was info available, but a bunch of self apointed gurus figured they could brak the laws of physics with ignorance and repeating irrelevant analogs ad nauseaum.

I thought that you would like analog solution, few pictures of you shooting arrows next to steam punk "time slicing apparatus" and you could sell your bows to hipster and your retirement would be secure.

That site has some interesting triggering info that you migh find entertaining.

Anyways, I found that when I was searching means to make external trigger amplifier for oscilloscope.

I never considered LED flash, I'm aware that there are LED video lights and flash lookalike videolighths available (seen them in supermarkets), but your mail made me think that LED can deliver pretty high output levels when pulsed. And I have (or had) old Metz that gave pretty good strobo, but with reduced intencity.

Hi Pekka, I haven't put too much time into thinking about triggering the camera yet. Since it will be, relatively speaking, a time exposure, with a strobe sequence occurring somewhere in the middle, it isn't critical.

More critical is firing the bow and the strobe together. I may just make something like a fixed crossbow stock that accepts different sized bows, and has a trigger for both arrow release and the strobe trigger.

Since the Arduino program and some simple controls will determine the number, frequency and duration of pulses, that should be straightforward, once triggered.

I worried about how to contain all this in a dark enclosure, but duhhhhhhhhh, just remembered we have this thing here in Vermont called "night"! So I can just do it outside with my present target setup.

My camera happens to be pretty good with low light -- a Fuji Finepix with up to ASA 3200 capability, and a night time fireworks setting -- which is just about ideal for a multiflash strobe shot, I believe.

I'm sensin' a soittin mismatch between what I'm trying to say and the picshure of what I be aimin for in this here pixure takin strobe thingy. Since a pigsure is woit a thousand woids, lemme jes sketch this out fer ya's. See, da arrow flight is irrelephant:

Distance traveled by arrow between any two strobe flashes divided by time interval of strobe flashes equals instantaneous velocity of arrow at that point. D/T = V

Difference between velocity at any two strobe flashes divided by time interval equals instantaneous acceleration.deltaV/T = A

Mass of arrow multiplied by instantaneous acceleration equals the force applied by the bow at that instant.MA =F

Charting the above force data over time will give a curve representing the true picture of the bow's firing cycle characteristics with an arrow of that mass.

Using arrows of different masses and charting those curves will give a picture of how mass affects that bow's performance.

The photos will also reveal if and when an arrow of a specific mass outruns the string, and many other points of interest to me.

The instantaneous velocity and acceleration/deceleration of the bow's limb tips can also be charted.

Synchronization (or lack) between the limb tips will be visible. Overshooting will be visiple. Terminal velocity of the arrow can be determined, etc. etc.

All these results will not be theoretical estimation, mathematical models, or opinions. They will be measurements of the actual power curve delivered by a particular bow, with all anomalies retained and displayed.